The 3rd generation Partnership Project (3GPP) organization specifies the architecture of mobile cellular networks such as like Global System for Mobile Communications (GSM) and Universal Mobile Telecommunications System (UMTS). Third-generation mobile systems (3G), based on WCDMA radio-access technology, such as UMTS, are being deployed on a broad scale all around the world. A first step in enhancing or evolving this technology was the introduction of the High-Speed Downlink Packet Access (HSDPA) and of the enhanced uplink, also referred to as High Speed Uplink Packet Access (HSUPA). In a longer time perspective it is however necessary to be prepared for further increasing user demands and be competitive against new radio access technologies. To meet this challenge, 3GPP has initiated a study item leading to Evolved 3GPP Packet Switched Domain, which is also known under the name Evolved Packet System (EPS). The EPS combines an Evolved Packet Core (EPC) network that is able to connect a new generation of an access network technology called Evolved Universal Terrestrial Radio Access Network (E-UTRAN) as well as the pre-successor of the E-UTRAN called Universal Terrestrial Radio Access Network (UTRAN). Another broadly used term for E-UTRAN (and having the same meaning) is Long Term Evolution (LTE). LTE is designed to meet the subscriber and network operator needs for high speed data and media transport as well as high capacity voice support to the next decade. A more detailed description of the EPS can be found in 3GPP Technical Specification (TS) 23.401; “General Packet Radio Service (GPRS) enhancements for Evolved Universal Terrestrial Radio Access Network (E-UTRAN) access”; v. 10.2.1 of January 2011, freely available through www.3gpp.org and incorporated herein by reference.
An LTE network architecture including network entities and interfaces between them is exemplified in FIG. 1. As can be seen in FIG. 1, the LTE architecture supports interconnection of different radio access networks (RAN) such as UTRAN or GERAN (GSM EDGE Radio Access Network), which are connected to the EPC via the Serving GPRS Support Node (SGSN). Some of the entities and interfaces are described below for facilitating the understanding of the exemplary embodiments of the present invention.
In a 3GPP mobile network, the mobile terminal 110 (called User Equipment, UE, or device) is attached to the access network via the Node B 120 (NB) in the UTRAN and via the evolved Node B (eNB) in the E-UTRAN access. The NB and eNB 120 entities are known as base station in other mobile networks. The NBs and eNBs 120 in UMTS and LTE also fulfil the functions of a base station, i.e. provide a wireless access interface for cellular wireless terminals. There are two data packet gateways located in the EPS for supporting the UE mobility—Serving Gateway (SGW) 130 and Packet Data Network Gateway 160 (PDN-GW or shortly PGW). The SGW terminates the interface towards the radio access networks, e.g. the UTRAN or the E-UTRAN. The mobility within one radio access network (UTRAN or E-UTRAN) is access specific. The mobility within the EPC is managed by the PGW. The mobility management in the EPC between the PGW and the SGWs can be based either on the Proxy MIPv6 (PMIP) protocol or on the GPRS Tunneling Protocol (GTP). The interface between the SGW and the PGW is called S5 and it can be based either on the GTP or the PMIPv6 protocol. The PGW further performs IP address allocation to the UE and packet filtering (e.g. deep packet inspection, packet screening) in order to map the UE's traffic to appropriate Quality of Service (QoS) level.
Assuming the E-UTRAN access, the eNB entity 120 may be connected through wired lines to one or more SGWs via the 51-U interface (“U” stays for “user plane”) and to the Mobility Management Entity 140 (MME) via the S1-MME interface. The 51-U interface is based on the GTP protocol and the S1-MME interface is based on the S1-AP protocol. The latter is specified in 3GPP Technical Specification (TS) 36.413; “S1 Application Protocol (S1AP)”; v. 10.0.1, January 2011, freely available through www.3gpp.org and incorporated herein by reference.
The SGSN 150 and MME 140 are also referred to as serving core network (CN) nodes. These network nodes maintain the context of the UE in the network, which means the security parameters, parameters used for the mobility management (e.g. in which are or cells the UE is camping, if the UE is reachable) and parameters used for the session management (SM) such as QoS parameters describing the communication sessions.
The SGW 130 routes and forwards user data packets, while also acting as the mobility anchor for the user plane during inter-eNodeB handovers and as the anchor for mobility between LTE and other 3GPP technologies (terminating S4 interface and relaying the traffic between 2G/3G systems and PDN GW). For idle state user equipments, the SGW terminates the downlink data path and triggers paging when downlink data arrives for the user equipment. It manages and stores user equipment contexts, e.g. parameters of the IP bearer service, network internal routing information. It also performs replication of the user traffic in case of lawful interception.
The MME 140 is the key control-node for the LTE access-network. It is responsible for idle mode user equipment tracking and paging procedure including retransmissions. It is involved in the bearer activation/deactivation process and is also responsible for choosing the SGW for a user equipment at the initial attach and at time of intra-LTE handover involving Core Network (CN) node relocation. It is responsible for authenticating the user (by interacting with the HSS). The Non-Access Stratum (NAS) signaling terminates at the MME and it is also responsible for generation and allocation of temporary identities to user equipments. It checks the authorization of the user equipment to camp on the service provider's Public Land Mobile Network (PLMN) and enforces user equipment roaming restrictions. The MME 140 is the termination point in the network for ciphering/integrity protection for NAS signaling and handles the security key management. Lawful interception of signaling is also supported by the MME. The MME also provides the control plane function for mobility between LTE and 2G/3G access networks with the S3 interface terminating at the MME 140 from the SGSN 150. The MME also terminates the S6a interface towards the Home Subscriber Server (HSS) for roaming user equipments.
The E-UTRAN comprises eNodeBs, providing the E-UTRA user plane by means of Packet Data Control Protocol (PDCP), Radio Link Control (RLC), Medium Access Control (MAC) and physical layer protocols (PHY) as well as control plane by means of Radio ResourceControl (RRC) protocol terminations towards the UE. The eNodeB (eNB) hosts the PHY, MAC, RLC and PDCP layers including the functionality of user-plane header-compression and encryption. The service the RLC layer provides in the control plane between UE and eNodeB is called Signaling Radio Bearer (SRB). In the user plane, the service provided by RLC layer between the UE and the eNodeB is called a Radio Bearer (RB) or Data Radio Bearer (DRB). The eNB also offers RRC functionality corresponding to the control plane. It performs many functions including radio resource management, admission control, scheduling, enforcement of negotiated uplink QoS, cell information broadcast, ciphering/deciphering of user and control plane data, and compression/decompression of downlink/uplink user plane packet headers. The eNodeBs are interconnected with each other by means of the X2 interface.
Amongst others, higher layer, i.e. Non Access Stratum (NAS), messages are carried by the RRC messages (e.g. using RRC Direct Information Transfer message) between the UE and the eNodeB. The Non Access Stratum is a functional layer running between the UE and the Core Network (CN) and located above the RRC. Furthermore, the NAS is the functional grouping of protocols aimed at Call Control (CC) for circuit switched voice and data, at Session Management (SM) for packet switched data and Mobility Management (MM) and at Short Message Services (SMS) for packet switched and circuit switched domains. The control messages the NAS layer generates are called NAS messages. Such messages are for example used to control Mobility Management, Session Management, SMS Transport and Call Management. NAS messages are transported transparently through the Access Stratum layers (layers 3-2-1, RRC, PDCP, RLC, MAC, PHY) that include the function and protocols to support the NAS transport. In order to send the initial non-access stratum message, the user equipment first establishes a Radio Resource Control (RRC) connection to the eNodeB over the air interface (Uu interface). During the RRC connection establishment the user equipment and eNodeB get synchronized and establish the Signaling Radio Bearers (SRB) that can be used for the transport of the non-access stratum messages.
The Access Stratum is the functional grouping of protocols specific to the access technique, in this case, the RRC, PDCP, RLC, MAC and PHY. It includes protocols for supporting transfer of radio-related information, for coordinating the use of radio resources between UE and access network, and for supporting access from the serving network to the resources provided by the access network. The Access Stratum offers services through Service Access Points (SAP) to the Non-Access Stratum (CN-related signaling and services), i.e. provides the Access Link between UE and core network, which consists of one or more independent and simultaneous UE-core network radio access bearer services, and only one signaling connection between the upper layer entities of UE and the core network.
When the UE is switched-off or not attached to the mobile network, the UE is in DEREGISTERED state. In DEREGISTERED state, no EPS Mobility Management (EMM) context exists and the UE location is unknown to an MME and hence it is unreachable by an MME.
When a mobile terminal (or user equipment, UE) is attached to the network, the UE is in the so called REGISTERED state, i.e. EPS Mobility Management context has been established and a default EPS bearer context has been activated in the network and in the UE. When the UE is REGISTERED to mobile network, the UE can be in two different connections management states: IDLE and CONNECTED state.
The UE is in IDLE state when there is no data for transmission and the radio resources are released, but the UE still has a valid IP configuration. A UE in IDLE state does not have a radio association (i.e. Radio Resource Control Connection, RRC) with the eNB, and therefore, there are no established signaling and data radio bearers. Further, there is no Non-Access Stratum (NAS) signaling connection between the UE and the network (e.g. to the MME) and also, there is no S1-U connection between the eNB and the SGW.
When the UE is in CONNECTED state and the network (usually the eNB) detects that the UE is not sending/receiving data for a certain period of time, the network (usually the eNB) decides to release the radio resources and the S1 connection. As a result, the UE transits from CONNECTED to IDLE state. Also the MME changes its internal state for the UE to IDLE and informs the SGW to release the S1-U connection to the eNB.
When the UE is in the IDLE state and uplink or downlink data or signaling (NAS signaling, e.g. due to the TAU procedure) needs to be exchanged between the UE and the network, the UE and the MME shall enter the CONNECTED state. In order to do it, the UE firstly needs to establish a Radio Resource Control (RRC) connection to the eNB over the Uu interface. During the RRC connection establishment the UE 110 and the eNB 120 get synchronized and establish the Signaling Radio Bearers (SRB) that can be used for the transport of the NAS messages. The RRC layer is a part of the so called Access Stratum (AS) including additionally the PDCP, RLC, MAC and Physical layers.
The above described IDLE and CONNECTED states are related to a NAS layer state diagram. On the other hand, in the AS layer the IDLE and CONNECTED states are also defined. The AS IDLE and CONNECTED states are similar but not completely analogical to NAS IDLE and CONNECTED states, i.e. if the RRC connection is established, the AS state is CONNECTED, otherwise if the RRC connection is released, the AS state is IDLE. Not always when the AS state is CONNECTED, the NAS state is also CONNECTED (e.g. for TAU procedure without active flag). The establishment of the RRC connection, and thus, the transition to AS CONNECTED state, is initiated by the UE, as only the UE can send “RRCConnectionRequest” message. The UE initiates the RRC connections establishment either due to the availability of uplink data or uplink signaling; or due to paging from the network in order to receive downlink data or downlink signaling.
For instance, when the UE has uplink data or uplink NAS signaling to send, the UE initiates the NAS Service Request procedure (described in 3GPP TS 23.401 cited above). The UE generates a “Service Request” message and triggers the AS to establish a corresponding RRC connection. The RRC establishment cause sent to the eNB in the “RRC Connection Request” message is set to “mo-Data” (mobile-originating data, meaning that the UE would like to send uplink data), “mo-Signaling” (mobile-originating signaling, meaning the UE would like to send uplink signaling) or to a special value “delay tolerant access” which is used when the application that triggered the session (connection) establishment is a delay tolerant application.
On the other hand, when the network (core network) has downlink data or downlink NAS signaling for the UE, the network initiates the Paging procedure.
Recently, 3GPP has started an activity on Network Improvements for Machine Type Communication (MTC). The service requirements have been described in 3GPP TS 22.368, v.11.3.0, October 2011, “Service requirements for Machine-Type Communications (MTC)”, freely available on www.3gpp.org, while the study of possible architecture solutions can be found in 3GPP TS 23.888, v.1.5.0, October 2011, “System Improvements for Machine-Type Communications (MTC)”, freely available on www.3gpp.org. The MTC terminals or MTC devices are characterized in that they are usually not operated by a human being. Rather, the communication peer is another machine such as a so called MTC server or another MTC terminal(s). As the MTC devices can be also mobile terminals as specified by the 3GPP, a more general notification like “UE” is also used throughout this description, so that the MTC device, terminal or UE are used interchangeable.
The MTC has some particular features which differ from the usual human-to-human communication. 3GPP tries to identify these particular features in order to optimize the network operations. These specifics are called “MTC features”. For instance, an MTC device typically sends or receives smaller amounts of data. Another feature of the MTC devices and 3GPP core network (CN) shall be the ability to allow an external server (MTC server) to trigger the MTC device to initiate a communication with the MTC server. This is enabled by a so-called “device triggering”. The Device Triggering is initiated by the MTC server and can be performed by different means.
The MTC devices could be a source of congestion/overload in the network. Thus, an efficient radio resource management (RRM) is necessary for the network, and in particular for the radio access network. As described above, when terminals (UEs) request a service from the network, they generate a rather high amount of signaling. If the amount of terminals is high as it is in the case of MTC devices (cf. TS 23.401 cited above, Section 4.3.17.2, “Overview of protection from Potential MTC Related Overload”), the network could be overloaded (congested) rather quickly before an appropriate admission (access) control could be applied.
The overload may occur since the total amount of signaling from a large number of terminals is a concern (is likely) in at least the two following exemplary situations:                when an application (running in many UEs) requests the many UEs to do “something” at the same time; and/or        when many UEs are roamers, if their serving network fails, then they shall likely all try to move into the local competing networks. This may potentially overload the not (yet) failed network(s).        
In general, in Release 10 (Rel-10) of the 3GPP standardization the congestion control mechanism in the network was extended with the NAS level congestion control. The introduction of NAS level congestion control was a result of the studies performed in 3GPP for the impact of Machine Type Communication to the network. It was concluded that the numerous MTC devices acting in simultaneous manner could cause congestion or overload to the network. To counter such overloads, 3GPP has specified several measures including:                a) Where applicable, UEs can be configured for enhancements as described in subsequent points; a post-manufacturing configuration can be performed remotely.        b) For mobile originated services, UEs configured for low access priority provide the E-UTRAN with information indicating that the RRC connection establishment is from a UE configured for low access priority when they establish connection to the E-UTRAN.        c) RRC signaling has the capability of providing ‘extended wait timers’ when rejecting messages from UEs configured for low access priority.        d) The MME can initiate rejection of RRC connection establishments in the E-UTRAN for certain subcategories of UEs. In addition, MME signaling or operation an management (O&M) signaling can trigger E-UTRAN to initiate Extended Access Barring for certain subcategories of UEs.        e) Overload messages from the MME to E-UTRAN are extended to aid the E-UTRAN in performing the functionality in the above points b), c) and d).        f) UEs configured with a long minimum periodic PLMN search time limit (cf. 3GPP TS 24.368 cited above) have an increased minimum times in between their searches for more preferred PLMNs.        
The NAS level Mobility Management control is applicable when many UEs initiate network access attempts almost simultaneously which could cause a congestion in the serving CN node (MME/SGSN).
Both Session Management (SM) and Mobility Management (MM) are considered as sublayers of the NAS (Non-Access Stratum) layer in the UE and in the MME/SGSN. Usually the MME/SGSN and UE store separate MM and SM contexts. Furthermore, the SM context is per PDP (Packet Data Protocol) or PDN (Packet Data Network) connection.
A dual-priority device is a device that is capable of running (executing) a low-priority application (a delay-tolerant application), a normal-priority application or both types of application simultaneously.
The dual-priority devices may thus alternatively or simultaneously execute applications with different priorities, i.e. may access service with a delay-tolerant characteristics (for instance an MTC application) or a normal service (for instance a telephony, streaming or internet access). The application and thus also the requirements of the service (and its priority) may change also during the connection. However, the radio access network and in particular the base station is only informed about the device priority at the time of the initial network access. Accordingly, the base station cannot appropriately perform the radio resource management tasks such as congestion or access (admission) control. The inconsistency between the real device priority of the terminal and the device priority relayed to by the base station may lead to wrong decisions in the situation of congestion. For instance, the base station may decide to release radio resources of a normal-priority terminal when the terminal initially accessed the network executing a low-priority MTC-application. Or, on the other hand, the base station may decide to allocate resources to a low-priority terminal when the terminal initially accessed the network executing a normal-terminal application.